Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and a photo-detector being integrated within a pickup. The photo-detector is used for detecting the reflected light of the laser beam when reading data from the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats, write-once optical media, as well as rewritable formats. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-ray disc (BD), which allows to store up to 50 GB on a dual layer disc. For reading and writing of a Blu-ray disc an optical pickup with a laser wavelength of 405 nm is used. On the Blu-ray disc a track pitch of 320 nm and a mark length from 2T to 8T and 9T is used, where T is the channel bit length, and which corresponds with a minimum mark length of 138-160 nm. Further information about the Blu-ray disc system is available for example from the Blu-ray group via internet: www.blu-raydisc.com.
New optical storage media with a super resolution near-field structure (Super-RENS) offer the possibility to increase the data density of the optical storage medium by a factor of two to four in one dimension as compared with the Blu-ray disc. This is possible by a so-called super resolution or Super-RENS layer, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from the optical storage medium. The Super-RENS layer is also called a mask layer because it is arranged above the data layer and for some specific materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, materials can be used for the mask layer, which show a higher reflectivity in the center part of the focused laser beam, e.g. InSb shows this nonlinear optical property. It can be assumed that the super-resolution effect is in particular based on a non-linear effect of some specific materials. Therefore, the Super-RENS effect allows to record and read data stored in marks of an optical disc, which have a size below the resolution limit of an optical pickup used for reading or writing the data on the disc.
A super resolution near field technique for recording and retrieving small marks beyond the optical diffraction limit is described for example by Tominaga, Nakano and Atoda in “An approach for recording and readout beyond the diffraction limit with an Sb thin film”, Applied Physics Letters, Vol. 73, No. 15, 12 Oct. 1998, which describe to use an Sb thin film as a super resolution layer.
A method for manufacturing of a master for an optical disc having marks of different widths is described for example in EP-A-0814464, which uses a laser providing a recording beam, and uses a modulator for varying the intensity of the recording beam for the production of the master. By providing a higher laser power for the shortest marks, the width of the shortest marks can be increased, to increase the legibility of the smallest width.
In “Random Signal Characteristics of Super Resolution Near Field Structure Read-Only Memory Disc”, Kim et al., Japanese Journal of Applied Physics, Vol. 45, No. 2B, 2006, pp. 1374-1378, measurements are described for a Super-RENS read-only disc having tracks with round pits of a first width, and of respective read-only discs having tracks with pits of a larger width. CNR results in dependency of the readout power are presented for pits having a length of 173 nm and pit widths ranging from 170 nm to 415 nm. Also shown are simulations for optical discs having pits with a width ranging from 64 nm to 160 nm.